Stay for beginning of party (4)
I believe the answer is:
prop
'stay' is the definition.
(I've seen this before)
'for beginning of party' is the wordplay.
'for' becomes 'pro' (I've seen this before).
'beginning of' indicates taking the first letters.
The initial letter of 'party' is 'p'.
'pro'+'p'='PROP'
(Other definitions for prop that I've seen before include "Support; object used on stage" , "Support (a rugby player?)" , "Hold (up), support" , "Seize and hold firmly" , "Person or thing providing support" .)